<!DOCTYPE html>   
 <html lang="en">   
 <head>   
   <meta charset="UTF-8">   
   <meta name="viewport" content="width=device-width, initial-scale=1.0">   
   <title>let命令</title>   
   <script>   
     {   
    var x = 20;   
    let y = 10;   
     }   
     console.log(x); // 20   
     console.log(y); // ReferenceError: y is not defined   
   </script>   
 </head>   
 <body></body>   
 </html>